1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and particularly to a sheet discharge unit for discharging a sheet having an image formed thereon to the outside of the apparatus.
2. Description of the Related Art
An image forming apparatus utilizing an electrophotographic process such as a copier, a printer, a facsimile machine or a complex machine of these includes an image forming unit for forming a toner image on a sheet by performing a predetermined image forming process, a fixing unit for heat-fixing the toner image on the sheet to the sheet, and a sheet discharge unit for discharging the heat-fixed sheet to the outside.
A sheet bearing a toner image heat-fixed thereto is easily deformed by heat and curled when being conveyed to the sheet discharge unit. Thus, the sheet discharge unit includes a corrugating member for making a sheet stiff or corrugating a sheet being discharged by a discharge roller and the like as a means for correcting a curl of the sheet. Since the creased sheet has the curl corrected, it is stacked on a discharge tray in an orderly manner.
Although sheets are prevented from being untidily stacked on the discharge tray by corrugating the sheets, sheet corrugation causes other problems in the case of forming toner images on both sides of sheets.
Specifically, the sheet discharge unit is so constructed that, when a sheet having a toner image formed on one side is fed again to the image forming unit via a predetermined path (so-called switch-back path), the sheet is conveyed to the switch-back path with a trailing end thereof with respect to a discharging direction in the lead after a part of the sheet except the trailing end with respect to the discharging direction is temporarily discharged to the outside by a discharge roller. Thus, the sheet is given stiffness or corrugated by the corrugating member. When passing a switch-back path, particularly a curved switch-back path, the corrugated sheet is conveyed to the image forming unit while being curved along a curved path in a corrugated state. This causes a conveyance trouble such as abnormal noise.
The following prior art is known as a technology for suppressing an occurrence of such a conveyance trouble. In a sheet discharge unit of the prior art, a discharge roller for discharging a sheet to the outside is used as a corrugating member, and the discharge roller is composed of a first roller including a plurality of first roller bodies arranged at predetermined intervals on a roller shaft and a second roller including a plurality of second roller bodies arranged at predetermined intervals on a roller shaft and facing the first roller bodies.
In the case of forming a toner image only on one side of a sheet, the first and second rollers are so relatively moved in an axial direction that the first roller bodies and the second roller bodies are offset from each other in the axial direction (i.e. are not in contact), thereby causing a curled sheet to pass between the first and second rollers, whereby the sheet is given stiffness while being discharged to the outside.
On the other hand, in the case of forming toner images on both sides of a sheet, the first and second rollers are so moved in the axial direction that the first and second roller bodies are opposite to and in contact with each other, thereby causing a part of the sheet except the trailing end thereof with respect to the discharging direction to pass between the first and second roller bodies opposite to and in contact with each other, whereby the sheet is conveyed to the switch-back path without being corrugated. After having toner images formed on and fixed to both sides, the sheet is corrugated while being discharged to the outside by the sheet discharge unit.
The above-described prior art can suppress an occurrence of a conveyance trouble when a sheet passes the switch-back path although a corrugating function is provided. However, the prior art requires a switching means for switching a state of the first roller bodies of the first roller and the second roller bodies of the second roller between a contact state and a non-contact state, which makes the construction more complicated. This results in not only cumbersome assembling, but also a cost increase due to an increase in the number of parts.